1. Field of the Invention
The present invention relates to a magnetoresistive head and a method of fabricating the same and, more particularly, to a magnetoresistive head used for reading information signals recorded on a magnetic recording medium in a hard disk device, a magnetic card device and a magnetic tape device etc. as a high density recording apparatus empolying a magnetic recording scheme and a method of fabricating the same.
2. Description of the Prior Art
In order to put a magnetic head employing magnetoresistance effect to practical use, a magnetoresistance layer (referred to as an MR layer hereinafter) must respond linearly to a magnetic signal field generated by a recording medium in a change of electric resistance.
In order to attain the linear response of the MR layer, a magnetic bias should be applied to the MR layer. Such magnetic bias is called as a linearizing bias.
Several approaches are known as means for applying the linearizing bias. As one method of the linearizing bias, a soft magnetic layer is arranged on the MR layer through a nonmagnetic metal layer. This method has advantages such as a good bias efficiency and a small shift in a bias point due to the sense current.
A conventional MR device employing such a structure is disclosed in Patent Application Publications (KOKAIs) 5-217123, 5-325138 and 5-182147, for example. The conventional MR device has a constitution shown in FIG. 1A, for instance.
Referring to FIG. 1A, an insulating layer 2 is formed on a substrate 1 made of a magnetic shield material. A soft magnetic layer 3, a nonmagnetic metal layer 4, and an MR layer 5 are formed on the insulating layer 2 in that order. A pair of connecting conductor leads 6a, 6b are formed on both ends of the MR layer 5 so as to put a sense area A therebetween. Antiferromagnetic layers 7a, 7b are formed between the connecting conductor leads 6a, 6b and the MR layer 5. The MR layer 5 is magnetized by an exchange interaction between the MR layer 5 and the antiferromagnetic layers 7a, 7b so as to be directed from one conductor leads 6a to the other conductor leads 6b. In addition, an insulating layer and a magnetic shield layer, both not especially shown in FIG. 1A, are formed on them.
In the above MR device, as shown in FIG. 1B, by causing a sense current I to flow into the area A, a magnetic field H1 is generated around the MR layer 5. The soft magnetic layer 3 is magnetized because of the magnetic field H1. Thus, a magnetic bias field H2 is generated by the magnetization of the soft magnetic layer 3 in the direction perpendicular to the direction of initial magnetization M5 of the MR layer 5. Then, a direction of the magnetization M5 of the MR layer 5 is changed by the magnetic bias field H2 of the soft magnetic layer 3. As a result, a characteristic curve of electric resistance to an external magnetic field shown in FIG. 1C can be derived. Therby, a change in electric resistance of the MR device responds linearly to a magnetic signal field generated by a magnetic medium (not shown).
In this magnetoresistive head, a requirement for a fine patterning of the device is caused, with progress of the high density recording by the magnetic medium. As a first approach for attaining a fine patterning of the magnetoresistive head, it can be considered that the MR device should be formed employing a thin film. For example, the soft magnetic layer 3 shown in FIG. 1A can be formed by the thin film. However, if the soft magnetic layer 3 is made thin, uneven coercive force is generated in the thickness direction. In order to improve this drawback, it has-been proposed in Patent Application Publication (KOKAI) 61-241993 that an amorphous insulating film can be used as a base film of the soft magnetic layer 3.
However, according to experiments of the inventors of the present invention, when the soft magnetic layer formed on an amorphous insulating Layer (for instance, Al.sub.2 O.sub.3) is made thin, there exists a certain thickness where a magnetic moment becomes zero regardless of the soft magnetic layer. Therefore, a linearizing bias cannot serve in the MR device which includes the soft magnetic layer of the certain thickness. Since the certain thickness generating zero magnetic moment is not constant and is varied in a range 20 to 30 .ANG. at the upper limit, for example, it is difficult to adjusts the magnetic moment by controlling the film thickness. In addition, a linearizing magnetic bias field becomes unstable. Ordinarily, the film thickness of the soft magnetic layer is set to be about 200 .ANG..
The problems associated with the soft magnetic layer are briefly as follows. In case the soft magnetic layer is formed relatively thickly so that the certain thickness generating a zero magnetic moment is negligible, the other drawback described above does not matter especially. But, if the soft magnetic layer is formed thinner than the certain thickness, still another drawback is caused such that a magnetic bias field which is applied to the MR layer is deviated from a designed value.